1. Field of tile Invention
This invention relates to a soft X-ray detector used as an image sensor for X-ray telescopes, soft X-ray microscopes, soft X-ray examining devices, etc.
2. Description of the Related Art
Soft X-ray detectors are known as microchannel plates (MCP), scintillation counters, imaging plates, semiconductor detectors, and soft X-ray solid-state image sensors such as charge-coupled devices (CCD).
FIGS. 1 and 2 are schematic views of a structure of the MCP. As evident from FIG. 1, an MCP 1 is composed of a bundle of numerous microchannels 2. Each of the microchannels 2 is made of a tiny glass pipe whose inner wall is formed as the combination of a resistor and a secondary electron emitter, and constitutes an independent secondary electron multiplier tube. Secondary electrons are emitted from the inner wall by radiation, such as soft X rays, or electrons incident on the microchannel 2. They are then accelerated by an electric field due to a high voltage applied to both ends of the microchannel 2, and travel through the microchannel while impinging on the inner wall in succession. In this process, the number of electrons is multiplied. The electrons are received on a fluorescent surface and then detected by the solid-state Image sensor such as the CCD.
The MCP is easy of use, because it has no sensitivity to visible light and brings about a desired spectral sensitivity by properly choosing a photoelectric converting surface. The MCP, however, is high In cost and needs a high voltage, with the result that its structure is complicated. Further, the thickness of the microchannel 2 is 12 .mu.m at a minimum, so that a spatial resolution is restricted in its improvement, with no less than about 30 .mu.m at present.
The imaging plate is a detector utilizing a photostimulated luminescence. Here, the photostimulated luminescence indicates that a substance first excited, for example, by soft X rays, is then Irradiated with Infrared light and emits light again. In the imaging plate, the substances exhibiting the photostimulated luminescence are distributed over the plate, and soft X rays, after exposure, can be read out by the two-dimensional scan of laser light. The resolving power depends on the diameter of a laser spot on reading out. Thus, although the spatial resolution is favorable, a readout mechanism is required. Consequently, there are defects that the structure is complicated and real time for signal detection is insufficient.
FIG. 3 shows the structure of a conventional CCD solid-state Image sensor used In the detection of soft X rays. In this figure, reference numeral 10 represents a photodiode in which an n.sup.- -type region 12 is provided on the surface of a p-type substrate; 13 a transfer gate provided in a region adjacent to the diode 10; 14 an aluminum tight shield provided in the upper region of the transfer gate 13; and 15 Incident radiation. The incident radiation 15 entering the solid-state image sensor reaches the photodiode 10, where it is converted photoelectrically and a resultant electric signal is transferred through the transfer gate 13. The aluminum light shield 14 is constructed so as to prevent a noise from entering the transfer gate 13. The conventional solid-state Image sensor, which is thus provided with the aluminum light shield 14, is complicated in surface structure and, as shown in FIG. 3, has needed a protective film 16 with a thickness of 3 to 5 .mu.m in order to maintain such a structure. Since the protective film 16 absorbs the most of soft X rays from the incident radiation, a very small amount of the soft X rays will reach the photodiode 10. For this reason, in the case where soft X rays are detected, the surface of the CCD is coated with a fluorescent film, such as Gd.sub.2 0.sub.2 S : Tb, so that the soft X rays are converted through the fluorescent film into visible light, which is then converted photoelectrically (for instance, Springer-Verlag, X-ray Microscope II, p. 127, 1987). This image sensor, however, is smaller in quantum efficiency than unity, low In conversion efficiency, and poor in sensitivity.
With devices using such a solid-state image sensor for detecting soft X rays, light of wavelengths except for the soft X ray region is emitted from a laser plasma radiation source or a radiation source of successive wavelengths such as spectral radiation (SR). As a consequence, for example, visible light is incident on the solid-state image sensor in combination with soft X rays, and forms the noise with respect to the soft X rays. Thus, as In soft X-ray microscopes for instance, when only soft X rays of short wavelengths are detected to derive an image for observation of high resolution, it is necessary to remove light other than the soft X rays incident on the solid-state image sensor by means of, for example, a spectroscope, in order to prevent the combination of the image with the visible light. With such a manner, however, the device is oversized and complicated, and hence loses the merit that the solid-state image sensor is used as the soft X-ray detector.
The above prior art can bring about only an image corresponding to light of a particular wavelength band predetermined In the design stage. This makes it disadvantageous to compare the image of soft X rays with that of visible light, for instance. In such a case, the MCP and the CCD can be switched for use. This, however, causes problems that the mechanism is complicated and at the same time, imaging becomes impossible.
Further, in order to obtain the image of light of a desired wavelength from white light, such as spectral radiation, it is necessary to monochromatize the light through a diffraction grating or a zone plate. In this case, there is the disadvantage that one optical element must be added. Moreover, an Imaging element, such as an objective lens using a multilayer film, although capable of making use of its wavelength choosing property, must be limited to the range of the designed particular wavelengths. Additionally, the multilayer film itself is high in cost.